The present invention relates to slope compensation for the stabilization of current-programmed converters and more particularly to the slope compensation of current programmed converters at high frequencies.
Hsu, Brown, Resnick, and Middlebrook in their paper "Modelling and Analysis of Switching DC-To-DC Converters In Constant-Frequency Current-Programmed Mode, IEEE Power Electronics Specialists Conference Jan. 1979, pp. 284-301 describe the method of slope compensation for the stabilization of current-programmed converters of duty ratios above 50%. The same technique can be used to improve noise immunity at any duty ratio. Circuitry is used to generate a periodic sawtooth ramp signal which is added to an input signal (the programmed threshold) which varies slowly compared to the sawtooth period. Referring to FIG. 1, a capacitor 5 can be inverted between the input stage shown as a differential amplifier 6 and the output driver 7. A constant current is forced through the capacitor 5, which results in a linear voltage ramp across capacitor 5. Capacitor 5 is then discharged periodically by a switched resistance 9, resulting in an additive sawtooth wave being presented to the output driver 7. If the output, minus the added ramp is to accurately follow the input signal over a wide range of output currents, then feedback must be employed. To avoid having the feedback negate the effects of the added ramp, the output can be sampled during the time that capacitor 5 is discharged using switch 11 and hold capacitor 13.
At switching frequencies above 1 MHz, several problems are encountered with the sampling technique described above. Discharging the capacitor 5 takes some time. The feedback sampling circuit must either delay sampling, which limits the operating frequency attainable, or else the sampling circuit feeds back a degraded signal, which includes the effects of the uncompleted discharge of the capacitor. If a delay is included to avoid feeding back a partially discharged capacitor voltage, then there is no convenient electronic indication of how long the delay should last. Either a conservative, long delay must be used, or else the sampling circuit could again feed back a degraded signal with the discharge of the capacitor not being completed.
If the command current used to charge the capacitor to generate the sawtooth waveform is to be controlled externally, then the command current must be great enough for off chip use, typically greater than a few microamps. If external resistors are used to generate the current, then the resistance must be small compared to board leakage paths and possible external contamination, typically having a resistance value of less than a few Megohms. Given such a current or current generated by a restricted range of resistance, for a given range of desired slopes, a minimum size requirement is placed on the slope-generating capacitor, which may result in a value impractical for on-chip implementation.
It is an object of the present invention to provide a slope compensation circuit which allows operation at higher frequencies of 1-2 MHz.
It is another object of the present invention to provide a slope compensation circuit which avoids feedback of an output voltage which includes the effects of a partially discharged slope capacitor without adding unnecessary delay.
It is a still further object of the present invention to provide a slope compensation circuit which limits the size of the on-chip capacitor in an integrated implementation while maintaining workable values of off-chip current and resistance to generate command currents for controlling the slope of saw tooth waveforms.